In the process of conventional wet-process phosphoric acid production, sulfuric acid reacts with phosphate rock to generate phosphoric acid and phosphogypsum. Phosphorus content in by-product phosphogypsum is generally more than 1.0%. The use of phosphogypsum with a phosphorus content of more than 0.5% is usually limited when it is used as chemical raw material or construction material. In many factories, phosphogypsum has been piled up like mountains and becomes a public hazard. Phosphogypsum has become a technical problem and a restrictive factor for the sustainable development of the phosphorus chemical industry.
At present, there are many researches on improving the technical process and optimizing the by-products, such as phosphogypsum, of the conventional wet-process phosphoric acid including the following patents.
Chinese patent CN103626143A (Application No.: 201310620402.7) discloses a method for preparing white gypsum, a by-product of wet-process phosphoric acid production. Firstly, phosphate rock powder (slurry) and phosphoric acid with a phosphorus pentoxide content (w %) of 16 to 32 react for 15 min to 60 min under stirring at a temperature range of 45 to 70° C. to generate a mixed slurry that contains solid impurities. The mixed slurry is subjected to continuous or discontinuous precipitation for 1.0 to 3.5 h, layered and separated, giving a thick slurry containing a mixed solution of phosphoric acid and calcium phosphate, as well as solid impurities. Under condition of stirring, sulfuric acid (40 to 98 (w %)) is added to the mixed solution of phosphoric acid and calcium phosphate and reacted for 10 min to 40 min. The resulting mixture is precipitated, layered and separated into phosphoric acid and white gypsum. By this method, phosphoric acid and white gypsum are obtained without the generation of phosphogypsum, eliminating the pollution from the piled phosphogypsum to air, soil and underground water. In addition, the by-product dihydrate white gypsum has high purity and high whiteness value. However, there are still acid-insoluble residues generated by this method, which is difficult to deal with. In addition, the dihydrate white gypsum needs to be subjected to dehydrate process or crystal modification process to obtain beta-gypsum or alpha-gypsum products with high added value.
Chinese patent CN102001636A (Application No.: 201010291898.4) discloses a method for preparing phosphoric acid with a wide-range concentration and clean gypsum through wet-process from low or medium grade phosphate rock. The method provides a new method for preparing wet-process phosphoric acid—hemi-dihydrate method, of which the by-product is high quality construction hemihydrate gypsum or functional dihydrate gypsum, making full use of low or medium grade phosphate rock. However, the method still discharges solid residues and dihydrate gypsum at a similar amount of the clean gypsum, which is hard to be used.
Chinese patent CN1421385A (Application No.: 02128116.5) discloses a method for preparing hemihydrate-dihydrate phosphoric acid. In the method, the precipitation rate of calcium in reaction tank is controlled and sulfuric acid is added at two steps. One part of sulfuric acid is added to acid-mixing tank, mixed with diluted phosphoric acid and then added to the second reaction tank; the other part of sulfuric acid is added to diluted phosphoric acid tank, and the concentration of sulfate ion in the diluted phosphoric acid is controlled to be from 8% to 10% SO42−. Alpha-hemihydrate gypsum is prepared firstly and then transformed to dihydrate gypsum. By-product of the method is still dihydrate gypsum, which is hard to be used directly.
Chinese patent CN103086335A (Application No.: 201310044529) discloses a dihydrate-hemihydrate method for producing wet-process phosphoric acid and at the same time obtaining by-product alpha-hemihydrate gypsum. The parameters for dihydrate process are: temperature of reaction tank is from 70° C. to 80° C., the duration is from 1.5 h to 3 h, the concentration of free sulfate ion is from 1% to 2%, and ω(P2O5) concentration of the wet-process phosphoric acid is from 35% to 39%. Parameters for hemihydrate process are: temperature of reaction tank is from 86° C. to 94° C., the duration is from 1 h to 2 h, the concentration of free sulfate ion is from 6% to 8%. ω(P2O5) concentration of the phosphoric acid prepared by hemihydrate process is from 10% to 15%, as an acid supplemental production of dihydrate process. The by-product, hemihydrate phosphogypsum, contains 5% to 7% crystal water, in which the mass percentage of free P2O5 is less than 0.4%, and the crystal form of which is alpha-hemihydrate phosphogypsum. In the method, the condition for dihydrate-hemihydrate crystal transformation is not strictly controlled, and no crystal transformation agent is used to control the aspect ratio of alpha-hemihydrate gypsum. Although alpha-hemihydrate gypsum product is produced, the strength of alpha-hemihydrate gypsum product is low, so the use of the product is limited. In addition, phosphorus content of the product is still relative high.
Hemihydrate gypsum (CaSO4.½H2O) powder is a kind of cementitious material. According to the content of impurity, color and external appearance, most of general alpha-hemihydrate gypsum is used to produce new type construction materials, such as cement flocculant and so on. Some gypsum products need to be made from high-purity, high-whiteness and high-quality alpha-hemihydrate gypsum, such as fiber gypsum board, gypsum plasterboard, gypsum suspended ceiling board, gypsum block, gypsum relief and lines and caulking, which are widely used in renovation and decoration. Therefore, an industrial large-scale production method that not only produces normal alpha-hemihydrate gypsum but also high-purity, high-whiteness and high-quality alpha-hemihydrate gypsum is an urgent demand of industrial production and market.
After search, no patent and report that provide industrial large-scale production method for preparing wet-process phosphoric acid and various kinds of high-quality gypsum at the same time is found.